[It's a bit ironic how "it's an excuse for a party" sounds better and more reasonable to him that a longer explanation about "making the best of being here", "it's still a happy day no matter what" or something similar.]

Hn... And I guess that since it's with your birthday as an excuse you really can play with admittance rules unlike with other parties where you could meet people you don't want to see. Pretty smart.

[Honestly, he hadn't thought of that.]

I hope the guys that weren't invited won't be in the mood to crash a party.

[That's probably the closest he could ever say to "hope you have a happy birthday party" and such. Even if he's probably still a jerk for even thinking of that.]